There is presently considerable demand for the reforestation of logging sites. However, young tree seedlings, such as spruce, do not always grow successfully once planted on such sites. A primary cause of failed spruce regeneration in Canada and the United States is the weed Calamagrostis canadensis (Michaux) Beauv., also known as bluejoint, reed grass and marsh reed grass.
Bluejoint is a grass species of the taxonomic family Graminaceae consisting of a complex of at least eight subspecies. C. canadensis is a perennial rhizomatous grass found in pine, spruce, and mixed-wood forests, open uplands and marshy sites throughout North America north of 35.degree. latitude (Lieffers et al "Ecology of and control strategies for Calamagrostis canadensis in boreal forest sites" Can. J. Forestry Research 23:2070-2077 (1993). The grass is very hardy, thriving in a wide variety of habitats including arctic environments, and in a wide range of moisture and nutrient regimes.
Bluejoint infestation is a serious problem in the disturbed open sites of reforestation areas where the weed can form continuous mats within about 3 to 4 years. At this stage the aboveground biomass may be up to 687 g/m.sup.3 (Lieffers et al, Can J For Res). Bluejoint grows profusely to a height of about 2 m or more, overwhelming young tree seedlings. In particular, bluejoint competes very strongly with the seedlings for nutrients, water and light. Furthermore, dense litter from the weed can cause snow-press and smothering problems, and can delay spring soil thaw. The litter also halts natural regeneration by preventing seeds from reaching the ground before desiccation. Bluejoint can cause seedlings to pitch-over and grow laterally. Moreover, weed litter can also provide a safe haven for seedling-eating mammals such as rabbits.
As a consequence of the above factors, C. canadensis can delay spruce development for up to 10 years, thereby seriously impairing reforestation efforts and timber production. A method of controlling the weed, especially in reforestation sites, is imperative.
There is currently much concern about the use of synthetic chemical herbicides, such as glyphosate, to control weeds. Consequently, the use of synthetic chemical herbicides is already barred or restricted in many areas of Canada and the United States.
Other non-chemical approaches to weed control have been considered including grazing by sheep, burning, mowing, mounding, trenching and mulching. These options are not generally viable and are impractical for reforestation sites. For example, burning is effective only with deep slash burning as the perennial rhizomes of the weeds regenerate readily. In fact, fire can have a rejuvenating effect on C. canadensis leading to large increases in biomass and seed production (Lieffers et al, Can J For Res 23:2070-77). However, deep burning has a devastating environmental impact. Mounding and trenching are also very destructive techniques. Sheep grazing techniques are expensive. While mulching is suitable for small private gardens, it is impractical and too labour intensive for large reforestation sites. Furthermore, complete weed eradication may be counterproductive, leading to wide-ranging problems including soil erosion, excessively high soil temperatures, and other negative phenomena.
The use of bioherbicides as biocontrol agents is therefore attracting considerable attention. Bioherbicides are typically endemic and applied inundatively to control undesired vegetation. In particular, mycoherbicides are bioherbicides which are fungal plant pathogens. The principal objective in dealing with weeds is to reduce competition from the weeds so as to allow the desired plants to grow freely. Accordingly, biocontrol of weeds addresses concerns about the effects of chemicals on environmental integrity and human health.
A variety of mycoherbicides have been proposed to control specific weeds. For example, Canadian Patent Number 1,224,055 (Watson et al, Jul. 14, 1987) describes the use of Colletotrichum coccodes for controlling velvetleaf and U.S. Pat. No. 4,643,756 (Cardina et al, Feb. 17, 1987) describes the use of C. truncatum for controlling Florida beggarweed. U.S. Pat. No. 4,776,873 (Caulder et al, Oct. 11, 1988) teaches a synergistic herbicidal composition comprising Alternaria cassiae and chemical herbicides for controlling sicklepod.
Inundative methods can achieve results ranging from partial to complete weed control or mortality, depending on the nature of the particular pathosystem, the formulation, and the virulence and concentration of inoculum. In particular, in order to control bluejoint, the bioherbicide should deplete carbohydrate reserves in the rhizomes in order to prevent rapid regeneration.
An effective bioherbicide causes disease in the target weed without adversely affecting the growth of crop plants. Factors which must be considered are that crop plants may be more susceptible to disease than a typical weed with its "wild type" resistance to pathogens. In addition, only certain parts or stages of the plant may be susceptible to the disease and major fluctuations in temperature, humidity, precipitation, or dew period can inhibit disease development. In terms of the pathogen, virulence may be difficult to maintain during inoculum production and storage.
It is an object of the present invention to provide an environmentally acceptable alternative to synthetic chemical herbicides for the control of weeds, such as Calamagrostis canadensis and related grasses.
It is another object of the present invention to provide a mycoherbicide for controlling C. canadensis and related grasses in coniferous reforestation areas.